In thermally assisted magnetic recording wherein a magnetic recording medium is irradiated with an evanescent light whereby the surface of the medium is locally heated and the coercive force of the medium is decreased, and writing is effected. This thermally assisted magnetic recording attracts attention as a magnetic recording system of the next generation which is capable of realizing a high plane recording density of approximately 1 T bit/inch2 or larger. In the case when the thermally assisted magnetic recording system is adopted, even when a recording medium having a coercive force of several tens kOe (note, 10 Oe equals to approximately 79 A/m) at room temperature is used, writing can easily be effected with a magnetic head having the currently available magnetic recording field.
Therefore a magnetic material exhibiting a high magneto crystalline anisotropy Ku of higher than approximately 106 J/m3 can be adopted for the recording layer. Thus, average particle diameter of magnetic crystal grains can be reduced to 6 nm or smaller while a high thermal stability is maintained. As such high Ku material, there can be mentioned, for example, a FePt alloy with an L10 type crystalline structure having a Ku of approximately 7×106 J/m3 and a CoPt alloy having a Ku of approximately 5×106 J/m3.
In the case when a FePt alloy with an L10 type crystalline structure is used for the magnetic layer, the FePt alloy crystal grains should preferably be (001)-ordered. The orientation of the FePt alloy crystal grains in a magnetic layer can be controlled by a layer formed underneath the magnetic layer. For example, it is described in Japanese unexamined patent publication H11-353648 that an FePt magnetic layer is (001)-ordered in the case when the FePt magnetic layer is formed above a (100)-ordered MgO underlayer with an intervening Cr underlayer.
It is described in Appl. Phys. Lett. 80, 3325 (2002) that a FePt magnetic layer is (001)-ordered in the case when the magnetic layer is formed on a (100)-ordered CrRu alloy underlayer. Further, it is described in J. Appl. Phys. 97, 10H301 (2005) that an FePt magnetic layer is (001)-ordered in the case when the FePt magnetic layer is formed above a RuAl underlayer having a B2 structure with an intervening Pt underlayer.
The magnetic layer of a thermally assisted magnetic recording medium is preferably comprised of, for example, an FePt alloy with an L10 structure having a high Ku value. To reduce the media noise of the thermally assisted magnetic recording medium, magnetic crystal grains in the magnetic layer should preferably be rendered fine. However, when an exchange coupling between magnetic grains is strong, adjacent magnetic grains are bonded together to form a large magnetic cluster resulting in enhancement of the media noise. Therefore, to reduce the media noise, the exchange coupling between magnetic grains must be weakened to minimize the formation of clusters having a large size, simultaneously with the reduction in size of magnetic crystal grains.
To minimize the formation of clusters having a large size, a grain boundary segregation material such as an oxide including SiO2, or carbon is preferably added into the magnetic layer to separate the magnetic crystal grains. However, when a limited amount of a grain boundary segregation material is added, the width of grain boundaries tend to decrease with a decrease in grain size. The decrease of the width of grain boundaries shortens the distance between adjacent magnetic grains, and consequently, locally strongly exchange-coupled grain pairs or grain groups often occur with the result of formation of magnetic clusters having a large size.
If a large amount of a grain boundary segregation material is added, the formation of magnetic clusters having a large size can be minimized, but, the addition of an excessive amount of a grain boundary segregation material tends to reduce the regularity of magnetic crystal grains having an L10 structure. Therefore, it is eagerly desired to effectively prevent the formation of magnetic clusters having a large size without addition of an excessive amount of a grain boundary segregation material.